In his tweet, Cook alluded to two police shootings that left two black men dead this week.

On Tuesday, Alton Sterling, 37, was killed buy Baton Rouge. La., police responding to a 911 call, and yesterday Philando Castile died from wounds sustained during a routine traffic stop in Falcon Heights, Minn. Cellphone video footage from both encounters surfaced on social media and the web, sparking outrage nationwide.

“Senseless killings this week remind us that justice is still out of reach for many,” Cook said. “We can and must do better.”

The message was in part a response to a tweet from civil rights leader and Georgia congressman John Lewis, who earlier on Thursday reminded followers that he was jailed for using a “white” restroom in Mississippi. Lewis attached his mugshot as a memento to the racial inequity he and many others believe still pervades America. He was released 55 years ago to the day.

During a book tour last year, Lewis visited Apple’s headquarters in Cupertino, Calif., where he talked one-on-one with Cook in a closed door meeting

Cook has long been an outspoken supporter of the civil rights movement, using his status in the technology community as a platform to speak plainly about gender and race inequality. His philanthropic work earned him an award from the Robert F. Kennedy Human Rights organization, where he now serves as a member of the board.